1. Field of the Invention
The present invention is directed to an arrangement for measuring and controlling the basic (static) magnetic field of an NMR tomography apparatus.
2. Description of the Prior Art
Current nuclear magnetic resonance tomography systems generally operate using the Fourier method as described in U.S. Pat. No. 4,070,611. The nuclear magnetic resonance signal is thereby phase-encoded in at least one direction. This method presumes an extremely high temporal stability of the basic (static) magnetic field that serves for the polarization of the nuclear spins because the image quality would otherwise be substantially deteriorated by artifacts (smears in the direction of the phase-encoding gradient). This is set forth in greater detail in the book by E. Krestel, editor, "Bildgebende Systeme fur die medizinische Diagnostik", 2nd edition, 1988, the section "Feldstabilitaet" on pp. 491 and 492. A field stability of up to less than 20-80 nT is required given the field strengths of 0.1-2.0 T as are currently standard for nuclear magnetic resonance tomography systems and the pulse sequences currently employed. This field stability must be maintained over a time span of a few ms up to several seconds (corresponding to a frequency range from a few tenths of a Hz up to a few tens of Hz); the demands decrease greatly at higher frequencies (or shorter times). Dependent on the strength of the basic field, this demand corresponds to a precision of 0.1 ppm-0.01 ppm and below.
This demand is true both of the field stability of the magnetic field generated by the basic field magnet itself as well as for external influences.
External noise sources, for example, are vehicles magnetized in the earth's magnetic field or in the stray field of the magnet that move in the proximity of the magnet or lines traversed by alternating current or by variable direct current (transformers, aerial contact lines of trains, etc.). Without special measures, such noise sources must be at a great distance from the location of the nuclear magnetic resonance tomograph apparatus in order to have no influence on its operations. A street car whose aerial contact line current is 500 A generates, for example, a noise field of 50 nT at a distance of two kilometers when it is assumed that the field drop-off is inversely proportional to the distance. In practice, it is hardly possible to find an installation location for a nuclear magnetic resonance tomography apparatus at which external noise influences remain within tolerable limits without special measures at the apparatus itself.
Various measures are known for avoiding field instabilities dependent on the type of magnet. A distinction must be made between the stability of the magnetic field generated by the apparatus itself and external disturbances. Permanent magnets must be temperature-stabilized so that they are adequately stable. Superconducting magnets in a standard short-circuit mode (field drop usually less than 0.1 ppm/hour) are inherently stable. The difficulty in the case of normally conductive magnets is keeping the supply current chronologically constant (to less than 0.1 ppm dependent on the field strength).
Superconducting magnets likewise have clear advantages with respect to external disturbances. Even if they do not have active stray field shieldings, they at least partially attenuate disturbances due to the Meissner-Ochsenfeld effect, by an approximate factor of 10. Given an active stray field shielding that is composed of an external shielding winding connected in series in an opposite direction to the current flow in the magnet, the shielding effect against external noise fields can be restored with a superconductor auxiliary winding composed of relatively few turns. Such an arrangement is disclosed by European Application 0 468 415. It has also been found that the refrigerated radiation shields in the superconductor cryostat attenuate field disturbances having frequencies higher than a few Hertz due to excited eddy currents rather well as a consequence of their good electrical conductivity.
By contrast, permanent magnets of the ring type, normally conductive air coil magnets in a Helmhotz arrangement as well as permanently magnetic or electromagnetic yoke magnets barely shield against external disturbances, particularly when they are constructed relatively open in order to achieve better patient accessibility. A yoke magnet having a single-sided yoke and an open structure is disclosed, for example, in U.S. Pat. No. 5,200,701.
An active noise field compensation is therefore usually required for magnet systems having permanent magnets or normally conductive magnets.
Such an arrangement is disclosed, for example, in U.S. Pat. No. 5,245,286. A sensor coil is placed around each pole shoe of the magnet, the respective magnet field being acquired therewith. By means of a spatially symmetrical arrangement of the sensor coils in the magnet and a series circuit, the influence of the pulsed gradients on the measured magnetic field is thereby eliminated. A compensation coil that surrounds the entire measurement compartment is driven dependent on a deviation of the magnetic field from a predetermined value that is acquired with the sensor coils. Neither dc offsets nor extremely low-frequency changes of the magnetic field, however, can be measured with sensor coils.
U.S. Pat. No. 4,234,950 discloses the use of MR probes for measuring field inhomogeneities of a magnet for nuclear magnetic resonance tomography apparatus. Use is made of the fact that the nuclear magnetic resonance frequency given a specific type of nucleus is strictly proportional to the applied magnetic field. Given this arrangement, however, the measurement of field inhomogeneities does not ensue during the image acquisition and in the absence of activated gradients. External field disturbances, however, can suddenly appear during the course of the pulse sequence. A control of the basic field only outside of the measuring cycles is thereby at most suitable for superconductor magnets since their cryostat shields shield against the higher-frequency parts of the noise fields.